After looking into this a bit more, we have a more broad issue with some vendors being finicky with redeemable items, such as tokens, parts, and certificates, and the Cargo Hold. This includes them turning away eager customers when there are duplicates in both the Inventory Window and Cargo Hold (which is what is likely to be the issue in this case). Additionally, the vendors have been known to taunt customers by telling them that wares are affordable due to the needed tokens/parts/certificates being in the Cargo Hold, but then not allowing for a purchase.
If players still want to give the vendors their business after such rude treatment (as TRUE-MANDALORIAN pointed out) taking redeemable items from the Cargo Hold to the Inventory Window will satisfy the vendor and allow you to purchase as intended.
I would like to apologize on these vendor's behalf. We are giving them internal reprimanding, though it may be a while before their behavior improves. In the meantime, we'll get this posted to the Known Issues today.
Unfortunately this isn't the case, as I've had the Speeder parts in my inventory from the start. I've also taken every Speeder part I have from my Cargo Hold (so any duplicates), brought it with me to Nar Shardaa and gotten the same message. I've then placed every Speeder part (including Explorer) back into my Cargo Hold, left it, then taken them out another day to find the same error message every time.
I hadn't thought of trading all the Parts across Legacy yet, but as you know the Pleasure speeder in particular won't have that option if it proves to have issue as well, Parts being Bind on Pickup and all.
Thanks for your support regarding this issue!